1. Field of Invention
The inventions relate in general to placing a substrate onto a surface of a substrate holder. They further relate to transferring a substrate from a first substrate holder to a second substrate holder using a transfer unit. The inventions further relate to a support system for supporting a substrate, a lithographic apparatus comprising such a support system and device manufacturing using such a lithographic apparatus.
2. Related Art
A lithographic apparatus applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
When devices are manufactured using lithographic apparatus, an important factor in the yield, i.e. the percentage of correctly manufactured devices, which is a function of the accuracy with which layers are printed in relation to layers that have previously been formed. Poor accuracy results in overlay. An overlay error budget will often be 10 nm or less. To achieve such accuracy, the substrate should be aligned to the mask pattern so as to be transferred with great accuracy.
In order to achieve good image definition and layer overlay the irradiated surface of a substrate should be positioned accurately on supporting surface, i.e. a substrate holder, and be kept on the substrate holder as flat and as stationary as possible during exposure. Generally, for this purpose, the substrate holder is provided with a plate comprising a plurality of protrusions, also referred to as burls. On such a substrate holder, a substrate can be placed so that its backside is in contact with the burls, all of which lie in a well-defined plane. By connecting aperture(s) in the substrate holder to a vacuum generating device, the backside of the substrate can be clamped securely against the burls. The use of burls in this manner ensures that only a fraction of the area of the backside is actually pressed against a solid surface; in this way, the distorting effect of any particulate contamination on the backside of the wafer is minimized, since such contamination will most probably be situated in the empty spaces between burls rather than being pressed against the top surface of a burl.
However, if the substrate is fixed to the substrate holder as described above, the substrate will bend over the burls. As a result, an image being exposed on the substrate will shift locally. When the substrate after development is again positioned on the substrate holder for a second exposure, due to a different position with respect to protrusions, the local image shift will be different during the second exposure than during the first exposure. Consequently, an overlay error has been introduced.
With the continual desire to image ever smaller patterns to create device with higher component densities, there is pressure to reduce overlay errors, which leads to a desire for improved placement of a substrate on a substrate holder provided with burls.